Several methods have been previously used to determine the percent composition of a fluid mixture. One such method uses two acoustic chambers, one chamber containing a reference fluid mixture with a known percent composition, and the second chamber containing a fluid mixture whose percent composition is to be determined. A fixed frequency acoustic wave signal is sent through both chambers and the phase shift or differential is measured between the signals emanating from the two acoustic chambers. The composition of the fluid mixture is then determined by the phase differential between the signal passed through the reference fluid mixture and the signal passed through the fluid mixture of unknown composition.
This acoustic phase differential method of determining the composition of a fluid has several disadvantages. One such disadvantage is the limited accuracy imposed on phase differential measurements by the equipment used to make such measurements. For example, the phase differential in typical gaseous fluids may only be a few tens of degrees, which is difficult to measure with a great degree of accuracy. In certain environments, such as in an aircraft, it is imperative that a fluid monitor give very accurate measurement irrespective of temperature changes that can produce temperature differences between the fluids in the two acoustic chambers, thus introducing errors in the phase differential.